Short-circuit exhaust hoods direct air from an air source toward an exhaust air intake within the recess of the hood to be exhausted together with a load from a fume source. The short-circuit system involves an air source that supplies and directs conditioned air, unconditioned air (typically referred to as make-up air), or a combination thereof into the exhaust hood recess in order to enhance capture and energy efficiency. Short-circuit exhaust hoods have been widely adopted in commercial kitchens due to at least two different motivations. First, it is believed that short-circuit exhaust hoods are more energy efficient than their non-short-circuit counterparts because part of the required exhaust air supply is satisfied with unconditioned air. Second, codes that required a minimum amount of air (in cubic feet per minute (cfm)) to be exhausted from kitchens could be circumvented by the use of short-circuit hoods.
However, most short-circuit hoods simply do not work very well, especially in view of their complexity and cost. In particular, short-circuit hoods have at least two air passages (e.g., one for exhausting fumes and another for introducing make-up air into the exhaust recess) and potentially more than two air passages (e.g., an additional one for introducing conditioned air into the vicinity of the exhaust hood to enhance employee comfort). Despite this added complexity, short-circuit systems have not been able to reduce the volume of conditioned air needed to achieve full capture and containment of a fume load under certain conditions. In fact, a short-circuit system may actually increase the amount of conditioned air that is exhausted. To operate effectively, an exhaust blower or fan for the exhaust hood must operate at a higher speed than if the short-circuit system was not present due to the need to remove not only the effluent-laden air but also the make-up air from the short-circuit supply. Make-up air may also increase turbulence in the vicinity of the fume source, which may increase the volume of conditioned air that is entrained in the effluent, thereby increasing the amount of exhaust required.